In the prior art, systems are known which make it possible to transfer LNG at sea between a supplier ship and a client ship. A transfer system of this type is described for example in U.S. Pat. No. 8,286,678.
U.S. Pat. No. 8,286,678 describes a system which comprises on the supplier ship side four flexible ducts, three of which make it possible to transfer LNG from the supplier ship to the client ship, and one of which makes it possible to extract natural gas in the gaseous state from the client ship to the supplier ship, in order to balance the pressures in the gas overlay of the tanks of the two ships. The system also comprises an articulated arm, fitted on a platform of the supplier ship, and a mobile chassis which is supported by said articulated arm. The mobile chassis supports the ends of the four flexible ducts, and comprises means which make it possible to connect them to ducts of the client ship. In addition, the mobile chassis is secured against the hold of the client ship by means of a sucker or electromagnet device.
Since LNG is inflammable in the presence of oxygen, the pipes of the client ship and the supplier ship are inerted and purged by sweeping by inert gas, implemented before and after the LNG transfer operations.
For this purpose, the inert gas is circulated through the flexible ducts of the ship, and is conveyed to the pipes of the client ship to be inerted. Consequently, the aforementioned inerting operations are particularly lengthy, since they make it necessary to inert the flexible ducts of the supplier ship along their entire length.
In addition, in order to be able to initiate the transfer of the LNG, the ducts of the client ship have to be put into temperature and pressure conditions which permit the transfer of the LNG without giving rise to substantial evaporations of natural gas. For this purpose, LNG is generally kept in the ducts of the supplier ship, or LNG is circulated in these ducts, in order to keep them in or put them into acceptable temperature conditions.
However, once the ducts of the supplier ship are swept by an inert gas, their temperature rises to above acceptable conditions. Thus, after the sweeping of the ducts of the supplier ship by inert gas, said ducts must be previously cooled before envisaging transfer of LNG without excessive evaporation.
Thus, the transfer conditions according to the prior art do not give entire satisfaction, in particular as far as the operations before the transfer of the LNG are concerned, i.e. the connection, inerting and cooling are particularly lengthy.